Gas liquefaction with work expansion of major feed portion



Jan. 2, 1968 H; KN P'P ETAL 3,360,944

GAS LIQUEFACTION WITH WORK EXPANSION OF MAJOR FEED PORTION Filed April 5, 1966' 2 sheets-sheet 1 FIG. 1

- I l I INVENTORS HELMUT KNAPP IRVING WEISS AGENT STEVEN J. MARKBREITERI GAS LIQUEFACTION WITH WORK EXPANSION OF MAJOR FEED PORTION Filed April 5, 1966 2 Sheets-Sheet 2 CFIG.2

- INVENTORS HELMUT KNAPP STEVEN J MARKBREITER IRVING WEISS AGENT United States Patent 3,360,944 GAS LIQUEFACTION WITH WORK EXPANSION OF MAJOR FEED PORTION Helmut Knapp, Yonkers, Stephen J. Markbreiter, Whitestone, and Irving Weiss, Brooklyn, N.Y., assignors to American Messer Corporation, New York, N.Y., a corporation of New York Filed Apr. 5, 1966, Ser. No. 540,272 12 Claims. (CI. 62-12) This invention relates to the liquefaction of gas and more particularly it relates to the liquefaction of natural gas which is normally composed of mostly methane and lesser amounts of heavier hydrocarbons such as ethane, propane and butane as well as nonhydrocarbons such as nitrogen, helium, carbon dioxide, hydrogen sulfide and moisture. While the process of this invention may be employed for the liquefaction of various low-boiling gases such as nitrogen, oxygen, hydrogen, helium, ethane, ethylene and propane, the illustrative embodiment of the invention herein disclosed involves the liquefaction of natural gas inasmuch as the liquefaction of natural gas is of great commercial importance in many parts of the world.

Since about 600 volumes of natural gas are reduced to one volume when natural gas is in liquid form, it is clearly advantageous to liquefy natural gas to enable storage in tanks or reservoirs of more econmical and practical design. In the long-distance transmission of gas by pipeline, it is important to operate under substantially constant high load factor. However, because of wide variations in demand for gas at different times of the year, it is desirable to store gas when demand is low and to use the stored gas together with gas transmitted by the pipeline to satisfy the demand at peak loads. For such peakshaving use, the process of the invention is of particular commercial attractiveness.

While several processes have been proposed for gas liquefaction and some have been used commercially, there has been a continuing need to find a process requiring a simplified plant in order to reduce the capital investment, maintenance cost and power consumption.

A principal object of this invention is to provide an improved process for the liquefaction of gas.

A further object is to simplify the equipment for liquefying natural gas for storge at nearly atmospheric pressure.

These and other objects and advantages of the invention will be evident from the description which follows.

In accordance with this invention, the liquefaction of gas supplied at elevated pressure is achieved by utilizing energy in the gas to generate refrigeration by expanding the gas with the performance of work and using the refrigeration to liquefy a portion of the gas for storage as liquid at nearly atmospheric pressure. Therefore, refrigeration from any external source such as the well-known cascade type of refrigeration system is avoided.

In the accompanying drawings, FIGURES 1 and 2 are flow diagrams of illustrative embodiments of the invention in which natural gas is liquefied for storage in a tank maintained at only slightly above atmospheric pressure so that the stored liquefied gas may be used to supplement pipeline gas during a period of peak demand.

Referring to FIGURE 1, a pipeline by way of branch line delivers natural gas (volumetric composition on dry basis being 87.5% methane, 9.2% other hydrocarbons, 2.7% nitrogen and 0.6% carbon dioxide including traces of sulfur compounds) at an elevated pressure of about 300 p.s.i.a. (pounds per square inch absolute) and a normal temperature of about 90 F. to water eliminator 11 wherein all but a trace of the original moisture (approximately 13 pounds per million standard 3,360,944 Patented Jan. 2, 1868 cubic feet of gas) in the gas is removed. Various means for water removal are known and may be used to dry the gas; as a specific example, the drawing shows dryer 11 as comprising two vessels 11A and 11B which are filled with pellets of a molecular sieve adapted to abstract moisture and sulfur compounds from gas passed through the bed of molecular sieve pellets in each vessel. Through control valves at the inlet and outlet of each of vessels 11A and 11B which are arranged in parallel, natural gas passes through one of said vessels for a period while the other of said vessels is closed off to natural gas flow and is undergoing regeneration, i.e., accumulated water from a prior period of use with natural gas is driven off from the molecular sieve pellets. As soon as the vessel through which natural gas is flowing has its bed of molecular sieve pellets near the point where the molecular sieve is no longer effective in abstracting moisture from the gas, the control valves are reset so that natural gas now flows through the vessel which has undergoine regeneration while the other vessel is placed on the regeneration cycle.

Natural gas with only a trace of moisture discharges from dryer 11 into line 12 from which a minor portion such as 18% of the gas flow is diverted into line 13 by adjustment of control valve 14. The gas stream entering line 13 is intended for liquefaction and storage in a tank maintained at only slightly above atmospheric pressure. Line 13 passes the gas to carbon dioxide adsorber 15 which may be of any known type. Specifically, the drawing shows adsorber 15 as comprising two vessels 15A and 15B each of which contains a bed of molecular sieve pellets adapted to adsorb the carbon dioxide as well as the residual trace of moisture in the gas. Like dryer 11, vessels 15A and 15B of adsorber 15 are provided With control valves at the inlet and outlet of each vessel so that the gas from line 13 may be alternatingly passed through one vessel to remove carbon dioxide and residual moisture from the gas while the other vessel is subjected to regeneration, i.e., adsorbed carbon dioxide and moisture are driven off from the bed of molecular sieve pellets which had previously been used to remove these impurities from the natural gas.

The thus purified gas flows from adsorber 15 through line 16 into heat exchanger 17 wherein it passes in countercurrent and indirect heat exchange relation with cooling streams which will be identified hereinafter. The gas in line 16 issues from exchanger 17 at a temperature of about 1l5 R, which is several degrees below the dew point of the gas in line 16, and discharges into separation drum 18 wherein liquefied ethane and higherboiling hydrocarbons originally present in the gas are separated from the still gaseous methane, the separated liquid hydrocarbons being discharged from separator 18 through line 19 and the gaseous methane being passed through line 20 to liqnefier 21. The methane flows through liquefier 21 in counter-current and indirect heat exchange relation to cooling streams hereinafter identified, discharging into line 22 in substantially completely liquefied form.

From line 22, liquid methane at a temperature of about -160 F. flows through su'bcooler 23 in counterourrent and indirect heat exchange relation to cooling streams hereinafter identified, exiting by way of line 24 at a temperature of about --200 F. The subcooled liquid methane at a pressure of about 280 p.s.i.a. is expanded through reducing valve 25 in line 24 and discharged into separation drum 26 at a pressure of p.s.i.a. Liquid methane collecting in separator 26 fiows through line 27 and is expanded through reducing valve 28 down to a pressure of about 15 p.s.i.a. maintained in storage tank 29* into which the liquid methane at a temperature of about 25 8 F. is discharged from line 27.

The flash vapor resulting from the reduction in pressure of the liquid methane discharging into separator 26 flows out through line 30, passing in succession through subcooler 23, liquefier 21 and heat exchanger 17 to give up its refrigeration to the incoming natural gas before this methane vapor is utilized in any desired manner such as fuel for the generation of electricity or for admixture with the gas of line 46 hereinafter described.

Similarly, cold methane vapor resulting from the expansion of liquid methane in line 27 into tank 29 and from boiling caused by heat leaking into tank 29 is passed from tank 29 by blower 31 and line 32 successively through subcooler 23, liquefier 21 and heat exchanger 17 to release its refrigeration to the incoming natural gas. Methane vapor in line 32 issuing from heat exchanger 17 may likewise be used for power generation or for any other desired purpose. It may be desirable to minimize the quantity of methane vapor resulting from the expansion of liquid methane in line 27 into tank 29 in which event line 27 may pass through heat exchanger 33 in countercurrent and indirect heat exchange relation to a small portion, usually not exceeding about of liquid diverted from line 27 through line 27A and expanded through reducing valve 27B to a pressure equal to that in line 32 so that line 27A may discharge into line 32. Thus, the refrigeration developed by evaporating the liquid in line 27A is used to further subcool the liquid in line 27 so that less vapor results when this liquid is expanded into tank 29.

The bulk of the refrigeration for the process, however, is supplied by the major portion or remaining 82% of the natural gas which was not subjected to liquefaction. This gas in line 12 is first cooled by passage through heat exchanger 17 in countercurrent and indirect heat exchange relation to the cooling streams therein, two of which, namely, methane vapor streams 3t] and 32, have already been described. The cold natural gas of line 12 issuing from exchanger 17 at a temperature of about 100 F. discharges into separation drum 34 wherein condensed ethane and higher-boiling hydrocarbons drop out of the cold gas and are withdrawn from separator 34 by way of line 35. The cold gas leaves separator 34 through line 36 and passes through turbo-expander 37, the expanded gas discharging into line 38 at a pressure of about 60 p.s.i.a. and a temperature of 175 F. The expanded gas of line 38 flows through liquefier 21 in countercurrent and indirect heat exchange relation to the methane undergoing liquefaction and discharges into separation drum 39. The liquid hydrocarbons in line 19 are expanded through reducing valve and enter separator 39 at a pressure of about 60 p.s i.a. Likewise, the liquid hydrocarbons in line 35 are expanded through reducing valve 41 and discharge into separator 33.

The vapor phase of the streams collected in separator 39 flows out through line 42 while the liquid phase is withdrawn by line 43. Both vapor stream 42 and liquid stream 43 pass as coolants through heat exchanger 17 in countercurrent and indirect heat exchange relation to the natural gas stream that is to be liquefied for storage in tank 29. Liquid stream 43 is vaporized while being heated in exchanger 17 and the resulting vapor discharges into line 44 into which also flows heated vapor stream 42 on leaving exchanger 17. The combined vapors of line 44 pass through centrifugal compressor 45 which is directly coupled to turbo-expander 37 thereby providing a balanced load on expander 37. The vapor stream is compressed to a pressure of about 82 p.s.i.a. by compressor 45 and is passed by line 46 to any desired point for utilization in power generation or in any other economic manner. To ensure control of the load on expander 37, compressed 'gas from line 46 may be passed through line 47 and expanded through reducing valve 48 for discharge into line 44 which feeds compressor 45. Whenever desired, some or all of the liquid stream in line 43 may be made to fiow through line 49, by opening control valve 4 50, into line 27 to join the liquefied gas for heating value control.

The liquid methane thus accumulated in tank 29 during periods of low or normal demand for pipeline gas is available for supplementing the pipeline gas in a period of peak demand exceeding the capacity of the pipeline. During such a period of peak demand, liquid methane is withdrawn from tank 29 by line 51 and pump 52 which pressurizes the liquid for flow through heater 53 where in the liquid is completely vaporized and the resulting methane vapor at elevated pressure is then discharged by line 54 into any desired distribution system. During periods of peak demand, methane vapor stream 32 leaving heat exchanger 17 may also be compressed and fed to the distribution system to help meet the peak demand.

It will be understood that more liquid methane could be made by the operation shown in FIGURE 1 if the pressure of the gas leaving compressor 45 and line 39 was lowered. However, both compressor 45 and line 3t) have been described as delivering gas at a pressure of about 82 p.s.i.a. to demonstrate that the invention can be used even when the portion of the gas which is not liquefied must be at a high pressure (below the elevated pressure at which natural gas is supplied to the process) for most economic utilization. While the foregoing example involves certain specific pressures, it is obvious that the pressure of the feed gas in line 111 and the pressure of the discharge gas in line 46 may vary over wide limits with each installation depending upon the available feed gas and the most valuable use for the discharge gas of that particular installation. However, it is essential to the process of this invention that the pressure of the discharge gas in line 46 be less than the pressure of the feed gas in line 10. Likewise, the pressure of the expanded major portion of the gas in line 38 must be less than the pressure of the compressed discharge gas in line 46 in order that compressor 45 may absorb the energy delivered by expander 37.

In FIGURE 2, elements corresponding to elements in FIGURE 1 bear the same reference numerals. lt'n the process of FIGURE 2, the pressures of both the natural gas supplied by line It) and the gas which is not liquefied and is discharged through line 44 are lower than, respectively, the pressures of the gas supplied by line 10 and discharged through lines 30 and 46 in FIGURE 1.

In FIGURE 2, natural gas from a pipeline is supplied by branch line 10 at an elevated pressure of 215 p.s.i.a. and a normal temperature of about 70 F. However, this gas is first compressed into successive stages to a higher pressure before undergoing moisture removal in dryer 11; in first-stage compressor 45A the pressure of the gas stream is raised to about 280 p.s.i.a. and in second-stage compressor 453 the pressure reaches about 375 p.s.i.a. A minor portion such as 22% of the gas leaving dryer 11 by way of line 12 is diverted by line 13 and control valve 14 for passage through carbon dioxide adsorber 15. Thence, the purified gas proceeds through line 16, heat exchanger 17, separator 18, line 20, liquefier 21, line 22, subcooler 23, line 24, reducing valve 25, separator 26, line 27, heat exchanger 33 and reducing valve 28 into tank 29 as already described more fully in connection with FIGURE 1.

The major portion of the gas discharging into line 12 from dryer 11 passes through heat exchanger 17, separator 34 and line 36 to first-stage expander 37A wherein the pressure of the gas is decreased to about 87 p.s.i.a. and its temperature is reduced to about l61 F. The expanded gas flows through line 38 into line 55 into which flash vapors from separator 26 also discharge after passing by line 30 through subcooler 23. The combined stream of line 55 is warmed in liquefier 21 to a temperature of about -l24 F. and then passed through secondstage expander 3713 wherein the pressure of the gas is decreased to about 35 p.s.i.a. and its temperature is again reduced to about -161 F. The gas leaving expander 37B flows through line 56 and liquefier 21 into separator 39 in the same manner that expanded gas flowed through line 38 and liquefier 21 into separator 39 of FIGURE 1. The vapor and liquid phases in separator 39 leave through lines 42 and 43, respectively, pass through heat exchanger 17 in which they give up refrigeration to incoming natural gas, and discharge into line 44 at a pressure of about 34 p.s.i.a. suitable to feed the gas distribution system in a residential area.

The embodiment of the invention shown in FIGURE 2 is suitable for any installation where the pressure of the feed gas in line is not sufliciently higher than the pressure of the discharge gas in line 44 to result in eflicient recovery of refrigeration. For this reason, to raise its pressure, the feed gas of line 10 is passed through twostage compressor 45A, 458 which absorbs the energy from two-stage expander 37A, 37B.

Comparing the processes of FIGURES 1 and 2, it will be noted that the work performed in expander 37 by the major portion of the natural gas leaving dryer 11 is utilized in compressor 45 to recompress principally the expanded natural gas, whereas the Work performed in the two-stage expander 37A, 3713 by the major portion of the natural gas leaving dryer 11 is utilized in two-stage compressor 45A, 45B to raise the pressure of all of the natural gas supplied to the process. In short, the process of this invention effects liquefaction of a minor portion, preferably not more than about 25 of a gas supplied at elevated pressure, preferably at least about 150 p.s.i.a., with refrigeration produced by expanding a major portion of the gas with the performance of work utilized in compressing a portion or all of the gas.

It will be clear to those skilled in the art that the broad principles of the invention may be embodied in variations of the expander-compressor arrangements shown in FIG- URES 1 and 2. For instance, the gas leaving compressor 45A of FIGURE 2 might flow directly to dryer 11 and compressor 45B might then be used to increase the pressure of the gas in line 44. Similarly, the two-stage expander-compressor of FIGURE 2 might be replaced by a single-stage expander-compressor in which event the natural gas in line 10 after passing through the compressor would flow directly to dryer 11 and the expanded gas flowing through lines 38 and 55 would discharge directly into separator 39. Conversely, the single-stage expandercompressor of FIGURE 1 might be replaced by a twostage expander-compressor in which event the partially compressed gas in line 46 would pass through the secondstage compressor and the partially expanded gas of line 38 after leaving subcooler 21 would be further expanded in the second-stage expander, would flow up through an additional passage in subcooler 21 and would discharge into separator 39.

In the operation of expanders, it is well known that the gas to be expanded should not be cooled to such a low temperature that during expansion the temperature of the gas will drop to a level at which either impurities, such as carbon dioxide, in the gas will solidify or liquefy, or the gas itself will begin to liquefy because the formation of liquid or solid within the expander will reduce the efiiciency of the expander. For this reason it is preferable to cool the major portion of the gas, prior to its expansion, to such a temperature usually in the range of about -65 F. to 130 F. that neither liquid nor solid will be formed in the expander during the expansion of the cooled major portion of the gas.

Inasmuch as the subcooled liquefied gas as a practical matter frequently must flow a considerable distance from the liquefaction plant to discharge into the storage tank and the liquid head within the storage tank provides resistance to the discharge of the liquefied gas, the pressure maintained in separator 26 is preferably at least about 35 p.s.i.a.

Those skilled in the art will visualize many other modifications and variations of the invention set forth hereinbefore without departing from its spirit and scope. Accordingly, the claims should not be interpreted in any restrictive sense other than that imposed by the limitations recited within the claims.

What is claimed is:

1. An improved process for transforming gas from a pipeline at elevated pressure to liquid for storage at substantially atmosphereic pressure solely with the aid of refrigeration derived from decreasing said elevated pressure of said gas, said gas containing moisture and another condensible impurity, which comprises treating said gas to effect moisture removal, dividing the treated gas into a minor stream and a major stream, removing another condensible impurity from said minor stream, cooling the thus purified minor stream to effect in turn liquefaction and subcooling of the liquid, reducing the pressure of the subcooled liquid to a pressure above atmospheric pressure to yield flash vapor and subcooled liquid phases, further reducing the pressure of the subcooled liquid phase while discharging it into a storage zone maintained at substantially atmospheric pressure, withdrawing vapor from said storage zone, passing the withdrawn vapor and the flash vapor phase in countercurrent and indirect heat exchange relation with said purified minor stream undergoing liquefaction and subcooling of the liquid, cooling said major stream to a low temperature which will permit expansion of the cooled major stream with the performance of work substantially without liquefaction of said cooled major stream, expanding said cooled major stream with said performance of work to a pressure above atmospheric pressure, passing the expanded major stream in countercurrent and indirect heat exchange relation with said purified minor stream undergoing liquefaction and with said major stream undergoing cooling prior to expansion, and utilizing said performance of work to compress at least a portion of said gas.

2. The process of claim 1 wherein the gas containing moisture and another condensible impurity is natural gas composed of mostly methane and containing moisture and carbon dioxide as another condensible impurity.

3. The process of claim 1 wherein the minor stream is not more than about 25% of the treated gas.

4. The process of claim 1 wherein a small portion of the subcooled liquid phase is vaporized by pressure reduction and the resulting vapor is passed in indirect heat exchange relation to the remainder of said sub-cooled liquid phase prior to its discharge into the storage zone.

5. The process of claim 1 wherein the performance of Work by the expansion of the cooled major stream is utilized to compress the expanded major stream.

6. The process of claim 1 wherein the performance of work by the expansion of the cooled major stream is utilized to compress all the gas prior to its treatment to effect moisture removal.

7. An improved process for transforming gas from a pipeline at elevated pressure of at least p.s.i.a. to liquid for storage at substantially atmospheric pressure solely with the aid of refrigeration derived from the decreasing said elevated pressure of said gas, said gas containing moisture and another condensible impurity, which comprises treating said gas to effect moisture removal, removing another condensible impurity from a minor portion not exceeding about 25 of the treated gas, cooling the thus purified minor portion to effect in turn liquefaction and subcooling of the liquid, reducing the pressure of the subcooled liquid to a pressure of at least about 35 p.s.i.a. to yield flash vapor and subcooled liquid phases, further reducing the pressure of the subcooled liquid phase while discharging it into a storage zone maintained at substantially atmospheric pressure, withdrawing vapor from said storage zone, passing the withdrawn vapor and the flash vapor phase in countercurrent and indirect heat exchange relation with said purified minor portion undergoing liquefaction and subcooling of the liquid, cooling the remaining major portion of said treated gas to a low temperature in the range of about 65 F. to -130 F., expanding the cooled major portion with the performance of work to a pressure above atmospheric pressure, passing the expanded major portion in countercurrent and indirect heat exchange relation with said purified minor portion undergoing liquefaction and with said major portion undergoing cooling prior to expansion, and utilizing said performance of work to compress at least a portion of said gas.

8. The process of claim 7 wherein the gas containing moisture and another condensible impurity is natural gas composed of mostly methane and containing moisture and carbon dioxide as another condensible impurity.

9. The process of claim 8 wherein the performance of work by the expansion of the cooled major portion of the treated gas is utilized to compress the expanded major portion of said treated gas.

10. The process of claim 9 wherein about 10% of the subcooled liquid phase is vaporized by pressure reduction and the resulting vapor is passed in indirect heat exchange relation to the remainder of said subcooled liquid phase prior to its discharge into the storage zone.

11. The process of claim 8 wherein the performance of work by the expansion of the cooled major portion of the treated gas is utilized to compress all the gas prior to its treatment to effect moisture removal.

12. The process of claim 8 wherein the performance of work by the expansion of the cooled major portion of UNITED STATES PATENTS 2,209,748 7/1940 Schlitt 62-38 XR 2,409,458 10/1946 Van Nuys 6238 XR 2,433,508 12/1947 Dennis 6238 XR 2,520,862 8/1950 Swearington 62-38 XR 2,779,174 1/1957 Vesque 6238 XR 2,822,675 2/1958 Grenier 62--38 XR 3,182,461 5/1965 Johanson 62-23 XR 3,312,073 4/1967 Jackson et a1. 6223 XR NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner. 

1. AN IMPROVED PROCESS FOR TRANSFORMING GAS FROM A PIPELINE AT ELEVATED PRESSURE TO LIQUID FOR STORAGE AT SUBSTANTIALLY ATMOSPHEREIC PRESSURE SOLELY WITH THE AID OF REFRIGERATION DERIVED FROM DECREASING SAID ELEVATED PRESSURE OF SAID GAS, SAID GAS CONTAINING MOISTURE AND ANOTHER CONDENSIBLE IMPURITY, WHICH COMPRISES TREATING SAID GAS TO EFFECT MOISTURE REMOVAL, DIVIDING THE TREATED GAS INTO A MINOR STREAM AND A MAJOR STREAM, REMOVING ANOTHER CONDENSIBLE IMPURITY FROM SAID MINOR STREAM, COOLING THE THUS PURIFIED MLINOR STREAM TO EFFECT IN TURN LIQUEFACTION AND SUBCOOLING OF THE LIQUID, REDUCING THE PRESSURE OF THE SUBCOOLED LIQUID TO A PRESSURE ABOVE ATMOSPHERIC PRESSURE TO YIELD FLASH VAPOR AND SUBCOOLED LIQUID PHASES, FURTHER REDUCING THE PRESSURE OF THE SUBCOOLED LIQUID PHASE WHILE DISCHARGING IT INTO A STORAGE ZONE MAINTAINED AT SUBSTANTIALLY ATMOSPHERIC PRESSURE, WITHDRAWING VAPOR FROM SAID STORAGE ZONE, PASSING THE WITHDRAWN VAPOR AND THE FLASH VAPOR PHASE IN COUNTERCURRENT AND INDIRECT HEAT EXCHANGE RELATION WITH SAID PURIFIED MINOR STEAM UNDERGOING LIQUEFACTION AND SUBCOOLING OF THE LIQWUID, COOLING SAID MAJOR STREAM TO A LOW TEMPERATURE WHICH WILL PERMIT EXPANSION OF THE COOLED MAJOR STREAM WITH THE PERFORMANCE OF WORK SUBSTANTIALLY WITHOUT LIQUEFACTION OF SAID COOLED MAJOR STREAM, EXPANDING SAID COOLED MAJOR STREAM WITH SAID PERFORMANCE OF WORK TO A PRESSURE ABOVE ATMOSPHERIC PRESSURE, PASSING THE EXPANDED MAJOR STREAM WITH SAID CURRENT AND INDIRECT HEAT EXCHANGE RELATION WITH SAID PURIFIED MINOR STREAM UNDERGOING LIQUEFACTION AND WITH SAID MAJOR STREAM UNDERGOING COOLING PRIOR TO EXPANSION, AND UTILIZING SAID PERFORMANCE OF WORK TO COMPRESS AT LEAST A PORTION OF SAID GAS. 